Sure, that would work fine with me, but I hesitate a bit with 15-5 because if you did caster-martial you'd have both 8th level spells and Extra Attack. Granted few games ever reach such a point, so it's a minor issue really.
I was discussing this with a player Sunday for my current game. He's a Rogue and planning to go Warlock eventually, but thinking not until Rogue 5. He knows that means 3 levels of Warlock before he can jump back to Rogue 6. Right now I think we'll probably run until 10-12th level, so he's looking at a 7/5 split at the end, maybe 8/4...